It is common practice in the coating art to produce metal sulfide films on a surface by vapor deposition. Of particular interest are zinc sulfide films on articles, such as bulbs and lenses, for special radiation transmission effects.
Sulfide vapors may be generated by thermal evaporation, as described in U.S. Pat. No. 3,024,381 (Dalton et al.). They may also be generated by electron beam bombardment, as proposed in U. S. Pat. No. 3,468,594 (Vogl et al.). Still another technique involves chemical reaction of metal vapors with a sulfide gas, such as hydrogen sulfide, to yield the desired metal sulfide. This procedure is described, for example, in U. S. Pat. No. 2,732,313 (Cusano et al.).
It has also been proposed to employ vapor deposition techniques in producing monolithic, metal sulfide bodies. For example, zinc sulfide vapors may be generated and deposited, usually on a glass surface. The "green" body, thus formed, may then be sintered. Current practice is to employ a chemical reaction process wherein separate sources of reactants are volatilized and reacted at relatively high temperatures. The vapors thus formed are deposited onto a substrate maintained at a lower temperature.
Deposition rates in such a procedure are very slow, e.g. 100 micrometers/hour. Thus, the process extends over days, or even weeks, thereby requiring extreme care in temperature control. The resulting body is porous, and must be subjected to high pressure and temperature in order to achieve transparency. As a consequence, the process is feasible only for very costly items, such as certain infrared transmitting lenses.
Extensive literature exists on the production of pure oxide bodies by a technology referred to as "soot casting". In general, this technology involves treatment of fluffy oxide particles, produced by oxidation of precursor compounds, such as chlorides and organometallics. The oxide particles are suspended in a colloidal dispersion, the colloid gelled by a gelling agent, and the gel dried and sintered. The procedure is described, for example, in the following U. S. Pat. Nos.: 4,541,855 (Scherer); No. 4,561,872 (Luong et al.); No. 4,574,063 (Scherer). Insofar as we are aware, this technology has not been explored beyond oxide materials.